The Digital Visual Interface (DVI) has become very popular in the consumer electronics arena for high definition video and audio with personal computers and in-home theater systems. These interfaces are used to convey signals from a video source, such as a display controller, to a display device, such as a computer monitor, via twisted shielded cable pairs covered with an over-braid shield.
Due to the off-the-shelf availability of hardware and software that support these interfaces, DVI presents the opportunity to provide the aerospace/defense industry with high-quality, high-definition video capabilities. However, the signaling format, cable/connector system specifications, and the resulting off-the-shelf cable assemblies for DVI were designed to meet the electromagnetic interference (EMI) requirements stated in Part 15 of the Federal Communication Commission (FCC) Rules and Regulations for digital devices, as well as those applicable under the Electromagnetic Compatibility (EMC) Directive 2004/108/EC of the European Parliament. The EMI requirements for defense/aerospace equipment are much more stringent than those applicable under FCC Part 15 and the EMC Directive.
Commercial DVI standards were developed for residential applications. Such commercial DVI standards require a DC connection between the source (transmitter) and load (receiver) ground to complete the circuit. This DC connection creates a current return path from the load to the source. The cable connection between the signal ground at the transmitter and the signal ground at the receiver according to the commercial DVI standards makes such standards incompatible with aerospace applications. This ground connection is achieved through internal shields covering the individual twisted wire pairs and through an over-braid shield (described in more detail below with reference to FIG. 1). These ground connections between the transmitter and receiver represent an impedance common to the internal DVI signal current path and an external circuit path. It is through this common impedance that high-level external currents induced by lightning couple excessive voltages to the internal DVI receivers and transmitters. It is also through this path that the high frequency DVI signal currents couple small voltages and currents to the external circuit which in turn acts as an antenna radiating high frequency electromagnetic fields which may interfere with sensitive aircraft navigation, communication and other avionic systems. In a reciprocal manner, external electromagnetic fields impinging on the external circuit cause currents to flow through the impedance common to the internal DVI circuit path coupling noise voltages onto the DVI signals potentially interfering with the DVI signal reception.
A commercial DVI is not intended for aerospace applications. To act as a differential interface that does not force current in the shield requires that the collective impedance presented to each signal by the driver, receiver and wiring be identical and requires that the signal currents flowing in the wire pairs be equal magnitude and opposite polarity. If this is so, the differential signal currents flowing in the shield flow only in extremely short paths transverse to the signal wire path. On the other hand, common-mode signals cause currents to flow in the shield parallel to the signal path and usually over a much longer length. It is the common-mode currents that are primarily responsible for cable radiation. To the extent that the impedance presented to the signal wires are not balanced and the drive levels are not equal and opposite, a common-mode signal will be generated.
The standard driver of a commercial standard DVI is not exactly a balanced driver. In this aspect, the DVI deviates to some degree from the balanced output impedance characteristic of the ideal differential interface driver. Imbalances in impedance and drive currents cause high frequency common-mode currents to flow in the ground path through the cable shields in response to the DVI signals. As indicated in the above, the ground path provides a common-impedance to couple DVI signals to the external circuit path causing the radiation of high frequency electromagnetic fields.
Commercial DVI cabling uses the shields of the internal twisted pair cables connecting the transmitter and the receiver as a current return path. Commercial DVI cabling has an external over-braid shield in addition to the internal twisted shielded pairs. This over-braid shield, as well as the internal DVI shield used in the twisted pair cables, are connected to a chassis ground at both ends of the link.
A conventional DVI receiver configuration is shown in FIG. 1. Referring to FIG. 1, a conventional DVI receiver 100A includes a DVI cable equalizer 110 that receives DVI signals via inner shielded twisted pairs 200A and 200B. The inner shielded twisted pairs 200A and 200B convey DVI signals along channels 1-4 from a DVI transmitter (not shown for simplicity of illustration) to the receiver 100A. As shown in FIG. 1, for example, there are four channels of signals, each conveyed through individual shielded twisted pairs. The DVI cable equalizer 110 recovers high frequency components of the DVI signals that may have become attenuated through transmission in the twisted pairs. After being processed by the DVI cable equalizer 110, the DVI signals are conveyed to DVI receiver processing circuitry 150 for further processing for display.
The inner shielded twisted pairs 200A and 200B are connected via pin connections to the receiver 100A. The inner shielded twisted pairs 200A and 200B are surrounded by an over-braid shield 250 which may be connected to the receiver 100A via an external connector 275 including a plug which mates with a receptacle of the receiver 100A. The inner shielded twisted pairs 200A and 200B and the over-braid shield 250 are connected to a chassis signal ground 125A. The DVI cable equalizer 110 is also connected to the chassis signal ground 125A. Although not shown, the over-braid shield 250, as well as the internal twisted pairs 200A and 200B, are connected to the chassis ground at both the receiving and transmitting ends of the link. This configuration forces the DVI return currents to use both the internal twisted shielded pair return as well as the over-braid as a return path, thus creating electromagnetic emissions. Such emissions can interfere with sensitive aircraft navigation, communication and other avionic systems.
Although the emissions produced by commercial DVI configurations are low enough to comply with emission limits for consumer products, such as FCC Part 15; such emissions are not low enough to comply with emission limits established for high-performance and/or safety-critical applications, such as aerospace applications, including but not limited to the emission limits established for aerospace applications in either MIL-STD-461 or RTCA/DO-160.
Previous solutions for more stringent requirements deviate from the DVI standard by transforming the interface between the controller and the display from single ended to differential. These solutions require modification from the standard at both the transmitting and receiving ends of the link.
In addition, defense/aerospace equipment is often also subjected to transients produced on the aircraft wires and cables, e.g., when lightning strikes the aircraft vicinity. The low voltage, high-speed circuitry of the DVI transmitter and receiver are difficult to protect from damage due to lightning induced transients in the aircraft cable when both the transmitter and the receiver are referenced to the equipment chassis and the aircraft structure. The high-speed receiver inputs limit the capacitive loading and imbalance introduced by transient voltage suppressor circuitry to extremely low levels to avoid communication errors due to frequency distortion and inter-symbol interference. The present state of the art silicon avalanche diodes used to protect against the effects of transient voltages do not clamp at sufficiently low voltages and sufficiently high currents and with sufficiently low capacitive loading to protect the DVI transmitter and receiver.
There are some available transient voltage suppressing devices with low capacitance capable of protecting low energy transients, such as electrostatic discharge (ESD). However, devices capable of dissipating the energy of lightning induced transients are not currently available. Since the DVI equipment on defense/aerospace platforms is often necessary for safe flight and/or mission accomplishment, damage from an occasional transient is not acceptable.
There exists a need for a solution for a DVI interface which reduces the effects of transients due to lightning strikes and reduces emissions to an amount which meets strict aerospace requirements but does not require a redesign of the transmitting and receiving ends of the DVI link.